In a digital signal transmission system, it is nescessary, inter alia, to detect loss of signal received by the receiving equipment, such signal loss stemming either from a failure of the transmitter equipment or from a failure of the transmission medium used between the transmitter equipment and the receiver equipment.
For example, one method of doing this is described in published French patent application No. 2 525 839, and consists in performing analog integration on the received signal followed by threshold comparison, with proper reception giving rise to a constant mean value at the output from the integrator while signal loss gives rise to the output signal from the integrator decreasing or increasing in level relative to said mean value depending on whether the loss of the digital signal appears as a constant logic level "0" or "1" at the input to the receiver equipment.
The present invention makes use of the fact that a digital signal receiver includes a circuit for reconstituting the receive clock signal. There are two ways in which a receive clock signal may be reconstituted, one of which essentially makes use of a local clock which is put into phase by the transitions in the received signal, while the other of which proceeds essentially by processing the received signal directly, and more particularly by performing spectrum processing thereon in order to extract a spectrum line corresponding to the desired frequency.
Unlike the first type of circuit, a circuit of the second type produces a reconstituted clock signal which is such that a loss of the received signal gives rise to an absence of pulses in the receive clock signal, even though this property has not been sought in the past, nor has it been used to solve a particular problem.
In contrast, the present invention takes advantage of this property to solve the problem of detecting received signal loss in a particularly simple manner.
Another object of the present invention is to provide a received signal loss detection circuit which is digital, and therefore easily integrated.
Another object of the present invention is to provide a circuit enabling recovery of the received signal to be detected after a certain number of observation periods on this signal marked by absence of signal loss.